1. Field of the Invention
The present invention relates to a process for producing an electrode for a nonaqueous electrolyte battery.
2. Description of the Related Art
A battery comprising a nonaqueous electrolyte solution and a negative electrode made of an alkaline metal can form a high voltage battery providing a voltage as high as not lower than 3 V and thus can serve as a high energy density battery. However, such a battery, if it serves as a secondary battery, is disadvantageous in that it is liable to shortcircuiting due to the deposition of dendrite of alkaline metal during charging and thus has a reduced endurance. Further, since an alkaline metal has a high reactivity, such a battery can be hardly provided with a sufficient safety. In order to overcome these difficulties for lithium battery for example, a so-called lithium ion battery has been devised which comprises a carbon-based negative electrode made of graphite or carbon on which dendrite of metallic lithium can hardly be deposited instead of metallic lithium and a positive electrode made of lithium cobaltate or lithium nickelate. This lithium ion battery has been used as a high energy density battery.
In order to improve the safety of a nonaqueous electrolyte battery, it is important to reduce the amount of free electrolyte solution present in the vicinity of the positive active material and the negative active material. Accordingly, the filling of pores in the positive active material layer and the negative active material layer with a solid electrolyte instead of free electrolyte solution is extremely effective for the improvement of the safety of the battery.
However, in a nonaqueous electrolyte battery such as lithium battery and lithium ion battery, the majority of lithium ions taking part in the electrode reaction in the charge-discharge reaction are not those dissolved in the electrolyte originally. Lithium ions released from the active material move through the electrolyte and then reach the opposite electrode. Thus, the movement distance of lithium ion is long. Further, the transport number of lithium ion in the electrolyte of a nonaqueous electrolyte battery at room temperature is normally not more than 0.5 while the transport number of proton and oxyhydroxide ion in the aqueous battery is close to 1. In a lithium battery and a lithium ion battery, the movement rate of ion in the electrolyte is governed by the diffusion of ion. Accordingly, if the pores in the electrode are filled with solid electrolyte instead of electrolyte solution, the diffusion rate of ion in the electrolyte is very low, deteriorating the high rate charge-discharge properties of the battery. Thus, practical battery properties cannot be obtained.
In order to overcome these difficulties, an attempt has been made to improve the high rate charge-discharge properties of batteries by filling the pores in the electrode with porous polymer electrolyte so that ions can be rapidly diffused through the free electrolyte solution in the pores. In this arrangement, the amount of free electrolyte solution can be reduced as much as the pores in the electrode are filled with porous polymer, providing a battery having a higher safety than the conventional batteries free of polymer electrolyte. Further, since ions can be diffused rapidly into the pores in the polymer electrolyte, sufficient battery properties can be obtained (U.S. patent application Ser. No. 08/637,600).
The polymer electrolyte to be filled in the pores in the electrode needs to be elastic enough to deform according to the volumetric expansion and shrinkage of the active material during charging and discharging. Thus, the porous polymer electrolyte is preferably a polymer gel obtained by allowing a polymer to swell with an electrolyte solution. An optimum process for the preparation of the porous polymer is a solvent extraction method which allows the formation of continuous pores consisting of uniform spherical pores (U.S. Pat. No. 4,539,256). The solvent extraction method is a method for the preparation of porous polymer which comprises immersing polymer paste in a second solvent insoluble in the polymer soluble in the solvent for the polymer paste so that the solvent for the polymer paste is extracted to form pores and the polymer is solidified. In the solvent extraction method, as the second solvent in which the polymer paste is immersed water, which exhibits a strong polarity, a good solubility in many polymers and an insolubility in many high molecular compounds and is inexpensive, is most suitable. In a nonaqueous electrolyte battery, it is undesirable that water enters in the battery. However, a battery having little water content and good properties can be prepared by a process which comprises immersing an electrode in water, sufficiently drying the electrode, inserting the electrode into a battery case, and then injecting an electrolyte solution into the battery so that the polymer is allowed to swell to form a porous polymer. In a nonaqueous electrolyte battery such as lithium ion battery, as a current collector there may be used a metal. However, many metals corrode with water having an excellent electronic conductivity. Thus, when such an electrode is immersed in water, the current collector exhibits a remarkably deteriorated electronic conductivity on the surface thereof. Accordingly, such a battery is disadvantageous in that the deterioration of the current collector properties causes deterioration of the battery properties. Thus, as the current collector there cannot be substantially used a metal such as aluminum and copper.
The present invention has been worked in the light of the foregoing problems. It is an object of the present invention to inhibit the corrosion of a current collector by water and hence inhibit the deterioration of the battery properties due to the deterioration of the properties of the current collector even if the current collector is made of a metal and the electrode provided with the porous polymer is prepared by immersing an electrode having the polymer paste held therein in water.
According to the first aspect of the present invention, a process for producing an electrode of a nonaqueous electrolyte battery, is comprised of the steps of: preparing an electrode comprising a metal as a current collector and having an active material layer; preparing a polymer paste having a polymer dissolved in a solvent soluble in water; allowing the polymer paste to be held in the electrode; and extracting the solvent from the polymer paste with an aqueous solution containing an alcohol, phosphorus or phosphorus compound.
According to the second aspect of the present invention, in the process according to the first aspect, the aqueous solution containing a phosphorus compound is an aqueous solution of phosphoric acid or phosphate.
According to the third aspect of the present invention, in the process according to the first and second aspects, the concentration of phosphorus or phosphorus compound in the aqueous solution is from 1xc3x9710xe2x88x927 to 1 mol/l.
According to the fourth aspect of the present invention, in the process according to the first aspect, the concentration of alcohol in the aqueous solution is from 1 to 70% by weight.
According to the fifth aspect of the present invention, in the process according to the first and fourth aspect of the present invention, the alcohol is ethanol or methanol.
According to the sixth aspect of the present invention, in the process according to any one of the first to fifth aspects, the metallic collector is made of aluminum or copper.